A laser consists of a pumped gain medium situated within an optical resonator. The pumped gain medium provides light amplification, and the optical resonator provides optical feedback, such that light circulates within the optical resonator along a beam path and is repeatedly amplified by the gain medium. The optical resonator (or laser cavity) may be either a ring cavity or a standing-wave cavity. Optical pumping and electrical pumping by current injection into the gain medium are two known pumping methods. The emitted light wavelength need not be in the visible part of the electromagnetic spectrum. One of the elements within the optical resonator acts as the output coupler, whereby a certain fraction of the circulating power is emitted from the optical resonator to provide the useful laser output. A partially transmitting mirror is a typical output coupler. For semiconductor lasers, the output coupler is typically an end face of a semiconductor gain chip, which may be coated to provide a reflectivity which optimizes performance. It is frequently useful for laser control to position one or more secondary output couplers in the optical resonator, so that a small fraction of the circulating optical power is emitted from the secondary output coupler(s) in such a way as to be easily separated from the main laser output beam emitted from the output coupler.
A tunable laser is obtained when one or more suitable tuning elements are combined with a gain medium that provides gain in a wavelength band of interest. The most common type of laser tuning element is a tunable optical bandpass filter inserted into the laser cavity. A bandpass filter has relatively low loss in a narrow wavelength band (centered at a center wavelength λc), and relatively high loss for all other wavelengths with significant gain. Since the laser emission wavelength is at or near the wavelength at which the net gain (i.e. gain—loss) is maximal, a tunable bandpass filter is a suitable laser tuning element, provided the variation of filter loss with wavelength is greater than the variation of gain with wavelength.
In order to make a semiconductor laser tunable, it is sometimes desirable to employ an external cavity geometry, to permit the use of tuning elements that cannot be present in a monolithic semiconductor laser cavity. As light makes a round trip within an external cavity semiconductor laser, light is emitted from a pumped semiconductor gain medium, passes through various optical elements, and impinges on the gain medium as a return beam. Semiconductor gain media typically include an epitaxially grown multilayer structure, and are classified according to the propagation direction of the emitted light. A gain medium is a surface emitter if the propagation direction is perpendicular to the plane of the layers. A gain medium is an edge emitter if the propagation direction is in the plane of the layers. Edge emitting semiconductor gain media typically include a single mode optical waveguide. An optical beam emitted from a single-mode optical waveguide has an amplitude and phase profile, referred to as the mode profile, which is determined by the waveguide. The amplitude and phase profile of the return beam is generally not exactly the same as the mode profile, and in such cases, not all of the return beam power is launched (i.e. coupled) into the gain medium waveguide. For example, if a certain power Pb impinges on the waveguide endface, only some lesser amount of power P0 is actually launched into the waveguide. The coupling efficiency η=P0/Pb depends on how close the return beam amplitude and phase profile is to the mode profile.
One kind of laser tuning element is an acousto-optic (AO) device. AO devices are described in textbooks such as J. Xu and R. Stroud, Acousto-Optic Devices: Principles, Design and Applications, Wiley, N.Y., 1992. An AO device is a device where an AO interaction is manifested. The AO interaction is a parametric three wave mixing process within a medium where an incident optical beam and an acoustic beam interact to generate one or more diffracted optical beams. Commonly employed AO media include quartz, TeO2, and Hg2Cl2. However, the AO interaction can occur to some extent in any material medium (i.e. anything except a vacuum). Suitable AO laser tuning elements are typically designed to ensure that the diffracted radiation consists essentially of a single beam, referred to as the diffracted beam or the first order beam. That portion of the incident optical beam which is not converted to the diffracted optical beam is the undiffracted beam, also referred to as the zeroth order beam. Typically, both zeroth order and first order beams are emitted from the medium. The incident beam and the zeroth order beam have the same optical frequency, which differs from the frequency of the first order beam by plus or minus the acoustic frequency. The zeroth and first order beams may also differ in other ways, such as propagating in different directions, and/or having different states of polarization. The acoustic beam in an AO device is typically generated by a transducer affixed to a surface of the medium, where applied electrical power at a suitable radio frequency (RF) acts to launch an acoustic beam having the same frequency into the AO medium.
Since the AO interaction is a parametric three wave mixing process, the incident optical beam is efficiently converted to the diffracted beam only for a narrow range of incident optical beam wavelengths, centered about some center wavelength λc at which the phase matching condition is met. The center wavelength λc of the AO interaction can be changed by changing the RF frequency applied to the AO device. Therefore, if we regard an AO device as an optical three port device, with one input and two outputs (zeroth order and first order), then transmission from input to first order output (first order transmission) gives a tunable bandpass filter, and transmission from input to zeroth order output (zeroth order transmission) gives the corresponding tunable notch filter (i.e. relatively high loss for a narrow range of optical wavelengths centered about some center wavelength λc, and relatively low loss for wavelengths outside this range). Although many different AO devices are known, such as AO tunable filters, AO deflectors and AO modulators, this general description (i.e. bandpass filter in first order transmission and notch filter in zeroth order transmission) is generally applicable to AO devices.
Acousto-optic devices were first used to tune dye lasers [e.g. as described in Streifer et al., Applied Physics Letters 17(8) p335 1970; Taylor et al., Applied Physics Letters 19(8) p269 1971; Hutcheson et al., IEEE Journal of Quantum Electronics, p 462 April 1974]. Subsequently, AO devices were used to tune other lasers where the gain medium has a broad bandwidth, such as fiber lasers [U.S. Pat. No. 5,189,676 Wysocki et al; U.S. Pat. No. 5,255,274 Wysocki et al; U.S. Pat. No. 5,812,567 Jeon et al], semiconductor lasers [Coquin et al., Electronics Letters 24(10) p599 1988; Koh et al., Proc. SPIE v3631 p98 1999; Zorabedian, IEEE Journal of Lightwave Technology 13(1) p62 1995], and titanium-doped sapphire lasers [U.S. Pat. No. 5,835,512 Wada et al 1998]. In addition, there are several reports in the patent literature where methods of tuning a laser with an AO device are disclosed, such as [U.S. Pat. No. 4,118,675 Rahn et al 1978] where the AO device simultaneously acts as both a beam deflector and dispersive element to force oscillation at a desired wavelength. GB 2,153,137 Hall et al. 1985, discloses the use of an AO beam deflector and separate dispersive element to tune a laser. U.S. Pat. No. 5,724,373 Chang 1998, discloses a method for AO laser tuning where the AO interaction provides a waveguide mode conversion device, and narrowband filtering is obtained by use of this AO device in combination with appropriate polarization optics within the laser cavity.
In all the above cited references the diffracted (i.e. first order, deflected or mode converted) beam is of primary importance for the tuning mechanism, and the undiffracted (i.e. zeroth order, undeflected or non-mode converted) beam is not utilized by the tuning mechanism. All of these references teach the use of an AO device in first order transmission to tune a laser. Since first order transmission through an AO device entails a frequency shift, much of the prior art cited above is concerned with the effect of the uncompensated frequency shift on laser operation, and/or on various methods of compensating for the frequency shift so as to eliminate its effect on laser operation.
One reference teaches the use of other than the first order beam (although not the zeroth order beam) in an AO interaction to tune a laser. U.S. Pat. No. 5,384,799 Osterwalder 1995, discloses the formation of a periodic refractive index perturbation within an AO device due to a standing acoustic wave. Osterwalder further discloses a tunable narrowband feedback mechanism into the laser cavity, based on the narrowband reflection created by the disclosed periodic refractive index perturbation.